The world-wide coverage offered by the eight ultimate Omega transmission stations operating in the low frequency range just above 10 KH.sub.z makes the Omega navigation system universally attractive. The basic theory of operation of an Omega navigation receiver is that several lines-of-position are calculated which place the receiver within one of several lanes which may be eight or more miles across. Knowing an initial position to define which lane is the correct position, it is possible to track vessel position by intersecting lines of position, even where lanes are changed, by accurately recording the changes in the vessel LOPs with time.
Each line-of-position within a lane results from determination of the phase difference between two Omega station transmissions as received at the vessel's position. It is thus necessary that at least three stations be provided in order to obtain two intersecting lines-of-position, although it is preferable to have four stations. The determination of the phase difference between received Omega transmissions is typically achieved by comparing zero crossing points of cycles of the received transmission. Because of variation in signal strength at the point of reception, it is desirable to amplify and hard limit the received signal so as to provide a generally squarewave signal. It has previously been important that the squarewave be accurately symmetrical in order to prevent errors from creeping into the detection of phase difference through variations in zero crossings. Hard limiters which are capable of this function are known, however, when faced with a dynamic range of, for example, 100 db over which the hard limiters must operate, it becomes expensive and difficult to achieve a symmetrical squarewave response to all possible signal strengths over that range.